drm/i915: Replace big nested if block with early return

Looks nicer.

Not functional change.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Add "No functional change" as requested by Jani.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>